A SafetyQuantity can be used to subtract a number from the available quantity.
For example, if there are 5 units available and the SafetyQuantity is 1, only 4 will be sent to Amazon. This can be set on a company level so that it defaults for all products, but it can still be overridden on a product level on AmazonProperties page.
Its important to realize that the safety quantity subtracts from the channels available quantity, so if there is an available quantity of 5 and you have a safety quantity of 5, a quantity of zero will be sent to the channel.
Each channel has a default company-level safety quantity and a product-level safety quantity.
On the channel's Settings page, there are two safety quantity settings - one for regular products and one for replenishable products. Typically, Safety Quantity can be set to "0" for replenishable products, because there is no strict inventory for replenishable products.
Another option you have is to set percentages of available qty that should be sent to each channel. Read more about that here.